1. Field of Invention
This invention relates to an apparatus and method for enhancing the isolation of a monolithic microwave integrated circuit (MMIC) crossbar or cross-point switch matrix with high isolation between unconnected paths.
2. Description of the Prior Art
It is known to use microwave switch matrices in communications systems which require reconfigurable routing of multiple input signals to various outputs. For example, switch matrices are used on satellites to provide interconnectivity among the multiple beam antennae. The cross-point or crossbar switch matrix is a particular class of switch matrix which takes the form of an orthogonal arrangement of N input and M output transmission lines which necessarily cross at M.times.N cross-points and which are terminated in matched loads, as shown in FIG. 1. A crossbar switch matrix and a cross-point switch matrix are identical to one another, the terms cross bar and cross-point being equivalent to one another. At each cross-point, the input and output transmission lines are interconnected by a switching element. By actuating the appropriate switching element each of the signals present at any of the N inputs can be selectively routed to any of the M outputs. The switching element can be realized in a plethora of ways. Presently, the most common method is to use an appropriate configuration of either PIN diodes, bipolar transistors (BJT or HBT) or field effect transistors (FETs, MESFETs, or HEMTs). Often, it is desired to simultaneously route a single input to more than one of the outputs. This has been accomplished by using a switching element which, when actuated, couples only a small fraction of the energy in the input signal, amplifies it and then recouples the signal to the desired output. In this way, the signal incident at a particular cross-point can pass relatively unattenuated to the subsequent cross-points and thereby can also be routed to other outputs in the same manner.
Present cross-point switch matrices are usually constructed in accordance with two approaches. The first utilizes orthogonal three dimensional arrangements of transmission lines, switches, amplifiers, and directional couplers that are fabricated and assembled using multilayer Hybrid Microwave Integrated Circuits (HMIC). The second utilizes Monolithic Microwave Integrated Circuit (MMIC) technology wherein the orthogonal transmission lines and the switching elements are fabricated on a single substrate using semiconductor processing techniques. The orthogonal input and output transmission lines in the MMIC implementation are electrically isolated by routing one of the intersecting lines over the other using metal airbridges.
The MMIC cross-point switch matrix has several significant advantages over the HMIC technique. MMICs are generally smaller in size and have improved reliability due to the fewer welded or soldered interconnections. However, the MMIC switch matrix exhibits poor isolation of undesired input signals to the various outputs. In the MMIC implementation, the microwave signals present on the input transmission lines radiate and are easily coupled to the output transmission lines which are in close proximity. This is especially true at the points where the transmission lines cross over each other by means of the metal air-bridges. Typically, the separation between crossing input and output lines is only a few microns directly underneath the air-bridge and hence the coupling can be quite severe (i.e. low isolation). For example, the typical isolation of two 50 ohm microstrip lines (on a 100 um thick GaAs substrate) which cross by means of an air-bridge is on the order of 35 dB at C-band, and 30 dB at K-band. This level of isolation is not satisfactory for many applications. One attempt to improve the isolation is described in U.S. Pat. No. 5,117,207 for a "Monolithic Microwave Airbridge" and a related paper by Power et al. "Broad Band Monolithic Cross Point Switch Matrices", IEEE Microwave and Millimeter-Wave Circuits Symposium Digest, pp. 127-130, 1990. This approach uses an air-bridge, which incorporates a grounded metal shield between the overpass and underpass transmission lines. In so doing, the isolation of the crossover is improved to 55 dB and 40 dB at C-band and K-band respectively. However, monolithic cross-point switch matrices built with this shielded air-bridge still only exhibit 45 dB of isolation between unconnected channels at C-band, because there is still significant stray coupling between other parts of the circuit. Much higher levels of isolation are achieved in HMIC technology by routing the input and output transmission lines on different substrate layers which are separated by one or more ground planes. It is primarily because of the poor isolation properties of the monolithic switch matrix that the HMIC switch matrices are currently preferred.